It has been conventional to pour molten metal into continuous casting molds for the production of steel slab, while maintaining the conditions of pouring, cooling and slab withdrawal so that the still liquid metal core within the embryonic casting, extends down into and below (or downstream of) the parallel walled zone of the mold (e.g. DE PS No. 887,990).
In such prior art methods, however, it has been difficult, especially at the start of pouring, to be sure that the liquid metal core within the casting always extends downwardly below the neck between the tapered, pouring zone of the mold into the zone where the walls of the mold are parallel. Unless this liquid core always extends downstream of the tapered zone, either localized bridges or an incompressible block forms at the neck, at which point the casting cannot be pulled through the mold, and the start-up of the casting operation has to be aborted. The risk of this happening arises from the fact that, when pouring starts, the incoming metal congeals rapidly on the cold walls of the mold and on the cold head of the dummy strip which is used to plug off the downstream end of the mold cavity and to lead the strip from the mold subsequently. In addition, at the start of pouring, the inrushing molten metal, which comes in through a pouring tube (sometimes referred to in the industry as a "submerged entry tube" even though it is not submerged at the start), if not properly controlled, will splash up along the sides of the mold where it may congeal and thereafter stick to the mold wall or otherwise interfer with the proper formation of the casting shell (i.e. the congealed superficial layer within the casting adjacent to the mold wall), and thereby eventually lead to surface defects or ruptures of the skin (break-out) as the casting proceeds further downstream. Another problem is that the antioxidation cover for the surface of the metal bath, which is required for both lubrication between the casting and the mold wall and for protection against oxidation of the liquid metal at the exposed surface of the melt at the top of the metal bath, can be applied to the surface of the bath only after the distribution ports of the pouring tube have become fully immersed in the melt. This is because the anti-oxidation and lubricating covering material (which is usually particulate), must not be allowed to mix with the casting metal, which would happen if the covering material were applied before the distribution ports of the pouring tube were immersed. It has been found that in these prior art devices and processes, the prelubrication within the mold tends to lose its effectiveness before the lubricating effect of the covering material can become operative thereby resulting in unacceptably high and uneven friction between the embryonic casting and the mold walls. If these aspects of the initial phase of pouring are not carefully controlled, either the casting has to be aborted, or a substantial length of the cast material at the start will be rendered unusable by virtue of the formation of hot spots, cold shuts, laps, or other surface defects. Accordingly, with the conventional apparatus and processes, at the start of pouring an undue burden is placed on the operating personnel rapidly to achieve maximum static head of the melt and the optimum balance between pouring rate, and withdrawal rate while at the same time covering the melt soon enough to avoid excessive friction between the congealed surface of the casting and the mold walls. This undue burden leads in turn to operational errors.
It is, therefore, an object of this invention to provide both apparatus and methods for the start-up of pouring metal, preferably steel, into the flared inlet end of continuous casting mold for the production of strip metal which method substantially relieves the operating personnel of these burdens by automatically shortening the time to reach optimum conditions and reducing the risk of the metal sticking to the mold wall. Another object is to provide apparatus and methods whereby the anti-oxidation and lubricating covering for the surface of the metal bath can be applied and become effective before of the prelubrication in the mold becomes inefficient..